Fix create container output messages when the image is not existed locally

Signed-off-by: Lei Jitang <leijitang@huawei.com>

when create a container with an image which is not existed locally and without tag,the output messages would confuse the user.For example:

$ sudo docker run -ti ubuntu /bin/bash
Unable to find image 'ubuntu' locally

I think the output messages "Unable to find image 'ubuntu' locally" should be "Unable to find image 'ubuntu:latest' locally" ,which is more exactly and in fact 'ubuntu' is not an image. Sometimes I has an image 
'ubuntu:14.04' locally,but forget the tag when create a container,the output "Unable to find image 'ubuntu' locally" is confused because I has repository "ubuntu" locally,and "Unable to find image 'ubuntu:latest' locally" 
can remind me that I may forget the tag.
relate #8868